Before you commit yourselves and jump into the deep end...
Why is the world 'overpopulating'? Are there groups/cultures/nations responsible for, or contributing to, overpopulation? How do we know?
Is not having children of your own going to have an affect on overall population numbers? Will adopting make a difference? What are the potential consequences that can arise from adoption?
Why is not having children and/or adopting the right approach? If this is a global problem, what is the likelihood that people around the world would adopt the same approach? (no pun intended)
Do you realise what you'd be giving up? Why is choosing to have a child of your own selfish? What does this say about the world?
I don't think having your own children is selfish, as long as you are doing it for good moral reasons.
I think overpopulation in more wealthy countries such as Australia has a lot to do with the introduction of vaccinations, a plentiful supply of fresh food and clean water, medical advancements and easier access to medical treatment which is resulting in a longer life expectancy.
In developing nations I would say it has a lot to do with lack of availability to birth control, higher child mortality rates (parents have more kids as they expect some of them may die), and because larger families means there is more hands to help out with labor, working on farms etc.
In Australia I don't think we have to worry about overpopulation too much at this point in time, but it is something that we should start to think about.
Of course not having children will make an impact on the population if enough people do it, just as China's one child policy has been successful in reducing their population.
I think if we ever had to take action on our population in Australia then the best way to go about it would be to adopt something similar to China's one child policy. People still get to experience giving birth, have their own biological child and can raise them as they wish.
I can't talk from experience, but I would say the love and dependence you receive from a child would make you feel loved and wanted regardless of whether they are adopted or your own birth child.